ABSTRACT The multifaceted nature of cancer results in a disease that is complicated and difficult to treat. One common characteristic of this disease is the ability of cancer cells to re-acquire stem-cell like traits resulting in aberrant gene expression and resistance to therapy. As the molecular networks that are frequently aberrant in cancer are also utilized by normal stem cells, it is critical to understand how these pathways and mechanisms function in stem cells and are harnessed by cancer cells to promote disease progression. Aberrant gene expression in tumors is often a result of oncogene expression or tumor suppressor inactivation. Tumor suppressors are particularly challenging to target as these proteins are often missing or present only at very low levels in tumor cells. Our lab has recently identified the tumor suppressor, ING4, a chromatin modifier and a transcriptional regulator containing a PHD finger domain, as a critical regulator of hematopoietic stem cell specification. ING4 has been shown to be inactivated in several types of human cancer, including breast, prostate, colorectal, ovarian and lung cancers, astrocytomas, and hepatocellular carcinomas. Loss of ING4 expression is associated with a poor prognosis but the mechanisms of tumor-suppressive functions of ING4 are not yet fully understood. A potential mechanism of tumor suppressor activity of ING4 is through its negative regulation of the NF-?B pathway, implicated in carcinogenesis, tumor progression and drug resistance. We found that Ing4 depletion in zebrafish stimulates NF-?B activity and NF-?B inhibition can rescue HSC deficiency caused by the loss of Ing4. Our proposed Specific Aims will use both genetic and pharmacological approaches, both in developing zebrafish and in ING4-deficient human tumor cells, to identify potential mechanisms for combating the loss of this protein in cancer. Aim 1 is to investigate the effects of Ing4 inactivation in zebrafish on gene expression and to determine if targeting of gene expression pathways that are upregulated upon loss of Ing4 would rescue the effects of Ing4 inactivation on HSC specification in zebrafish. Aim 2 is to determine if small molecule inhibitors of the NF-?B and other upregulated pathways would rescue the effects of Ing4 loss of function in zebrafish. Aim 3 is to identify genes and pathways, inhibition of which remediates cellular effects of ING4 inactivation in mammalian tumor cells, by using the results of the analysis in zebrafish and of a genetic screen conducted in ING4-deficient human tumor cells. The success of the proposed study will serve as the basis for identifying drugs that could remediate phenotypic effects of the ING4 deficiency, with the ultimate goal of developing therapeutic strategies for cancers where ING4 has been inactivated.